Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells per device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected to one another because high power and large capacity are necessary for the middle or large-sized devices.
Preferably, the middle or large-sized battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing costs of the pouch-shaped battery are low, and it is possible to easily modify the shape of the pouch-shaped battery.
In order for the middle or large-sized battery module to provide power and capacity required by a specific apparatus or device, it is necessary for the middle or large-sized battery module to be configured to have a structure in which a plurality of battery cells is electrically connected in series to each other or in series and parallel to each other and the battery cells are stable against external force.
Meanwhile, the battery cells constituting the middle or large-sized battery module may be secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from such high-power, large-capacity secondary batteries during charge and discharge of the secondary batteries. If the heat, generated from the unit battery during charge and discharge of the unit battery, is not effectively removed from the unit battery, the heat accumulates in the unit battery with the result that deterioration of the unit battery is accelerated. According to circumstances, the unit battery may catch fire or explode. For this reason, a battery pack for vehicles, which is a high-power, large-capacity battery, needs a cooling system to cool battery cells mounted in the battery pack.
In a middle or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the non-uniformity in performance is the non-uniformity of cooling between the battery cells. For this reason, it is necessary to provide a structure to secure cooling uniformity during the flow of a coolant.
FIG. 1 is a vertical sectional view typically showing a conventional middle or large-sized battery pack. Referring to FIG. 1, a battery pack 40 is configured to have a structure including a battery module group 11, an electronic member 30 mounted to one side of the battery module group 11, specifically to the left side of the battery module group 11 on the drawing, and a pack case 20, in which the battery module group 11 and the electronic member 30 are mounted.
A coolant introduction part 22 is defined below the battery module group 11 and a coolant discharge part 24 is defined above the battery module group 11. A coolant, introduced into the coolant introduction part 22 through a coolant inlet port 21 as indicated by an arrow, sequentially passes through the battery module group 11 and the coolant discharge part 24 while cooling unit cells of the battery module group 11 and is then discharged out of the battery pack through a coolant outlet port 23 as indicated by another arrow. Although not shown, a coolant flow channel is defined between the respective unit cells, which are vertically stacked, of the battery module group 11.
In the above structure, however, a space of the coolant flow channel is limited and, therefore, differential pressure is generated. As a result, it is difficult to achieve uniform cooling between the battery cells. In addition, since the coolant discharge part 24 and the coolant outlet port 23 constitute a continuous duct structure, the coolant having passed through the battery module group 11 is discharged out of the battery pack without being circulated in the battery pack except the interior of each battery module. As a result, it is difficult to cool heat generating elements, such as a battery management system (BMS) and a bus bar, except the battery cells.
Consequently, there is a high necessity for technology to fundamentally solve the above problems.